1. Field of Invention
The present invention relates to yellow inorganic pigments comprising oxides of alkaline earth, praseodymium, and transition metals. More particularly, the present invention relates to yellow inorganic pigments having the general formula: APr2MoTmxO6+η or Pr2MoTmxO6+η (Tm=Ti or Zr and x=0 or 1) wherein A is selected from one or more alkaline earth metals [A=for example, Mg, Ca, Sr or Ba, or combinations thereof], and wherein η has a value commensurate with the value of x and sufficient to satisfy the valencies of the formula. The present invention also relates to a process for the preparation of yellow inorganic pigments of general formula APr2MoTmxO6+η or Pr2MoTmxO6+η (Tm=Ti or Zr and x=0 or 1) wherein A is selected from one or more alkaline earth metals [A=for example, Mg, Ca, Sr or Ba], and wherein has a value commensurate with the value of x and sufficient to satisfy the valencies of the formula. The yellow inorganic pigment is well suited for coloring applications of a wide variety of substrates for example paints, varnishes, plastics, ceramics etc.
2. Background
Inorganic pigments are widely used in various applications such as paints, inks, plastics, rubbers, ceramics, enamels, and glasses. Unfortunately, the majority of the colorants currently employed for the above applications contain toxic metals (e.g., Cr, Co, Cd, Hg, Pb, etc.) that can adversely affect the environment and human health if critical levels are exceeded (further reference may be made to “High Performance Pigments,” ed. by H. M. Smith, Wiley-VCH, Weinheim, 2002, incorporated herein by reference). Thus, a serious need arises to search for environmentally friendly colorants for the replacement of toxic inorganic pigments, for use on industrial scale and which otherwise avoid the above disadvantages and drawbacks to date characterizing the state of this art.
Yellow is a particularly important color in the ceramic pigment field and the consumption of the yellow pigment exceeds that of any other colored pigments. There are three important yellow pigment families: tin vanadia yellows (further reference may be made to DCMA 11-22-4), praseodymium zircon (further reference may be made to DCMA 14-43-4), zircon vanadia yellow (further reference may be made to DCMA 1-01-4). It will be appreciated that reference to a particular DCMA number can be found in the art, for example, in DCMA, “Classification and Chemical Description of the Complex Inorganic Color Pigments,” Third Edition, 1991, published by the Dry Color Manufacturers' Association, USA, which is incorporated herein by reference. Other yellow ceramic pigments commonly used such as Pb2Sb2O7, PbCrO4, CdS are now being expelled from the market because of their toxicity (further reference may be made to J. A. Badenes, M. Llusar, M. A. Tena, J. Calbo, G. Monros, J. Eur. Ceram. Soc. 2002, 22, 1981-1990, which is incorporated herein by reference).
The use of praseodymium doped zirconium silicate crystals as a pigment for use in ceramic glazes was disclosed by C. A. Seabright in U.S. Pat. No. 2,992,123, July 1961. Since that time, there have been numerous patents issued for praseodymium doped zircons for ceramic applications and now it is manufactured worldwide. Stable pigment particles comprising praseodymium doped zirconium silicate particles, at least about 50 percent by volume of which range from 0.2 to 2 um size has been disclosed in U.S. Pat. No. 5,316,570, May 31, 1994, which can be used for plastics and paints.
U.S. Pat. No. 5,275,649, Jan. 4, 1994, describes a process for the preparation of environment-friendly inorganic yellow pigments based on praseodymium zircons (ZrSiO4:Pr), which can be applied to ceramics because of its thermal stability. However, this pigment requires high temperature calcination (˜1300° C.).
Non-toxic yellow/orange pigment compositions well suited for the coloration of a wide variety of substrates, for example paints, varnishes, ceramics, etc. comprise a major amount of zirconium oxide and an additive amount of cerium, praseodymium and or terbium values in the form of oxides have been reported in the U.S. Pat. No. 5,560,772, Oct. 1, 1996. The calcinations are generally carried out under air atmosphere by heating at least to 1550° C.
Oxonitrides with a perovskite of the general formula LnTaON2, where Ln is a rare-earth element, and which exhibit yellow-orange to reddish brown in color with an enhanced brightness, are produced in the presence of mineralizing agent from the series of alkali metal or alkaline earth halides, by annealing a powder mixture consisting of a Ta (V) compound and a Ln compound in a reducing atmosphere of ammonia (Reference may be made to U.S. Pat. No. 5,693,102, Dec. 2, 1997; M. Jansen and H. P. Letschert, Nature 2000, 404, 980-982). However, in the preparation of these perovskites, it is necessary to heat the starting material in the flow of toxic and inflammable ammonia gas for a long period (20-60 hrs.).
U.S. Pat. No. 6,419,735, Jul. 16, 2002, discloses a process for the preparation of samarium sesquisulfide pigment. The process consist of reacting samarium, trivalent rare earth metal, and alkali metal or alkaline earth metal compounds with a gaseous mixture of hydrogen sulfide and carbon disulfide. The compositions of the invention exhibit a strong yellow color.
U.S. Pat. No. 5,336,312, Aug. 9, 1995 describes a process relating to the yellow colorants represented by the formula Bi2Ax−1DxOy wherein: A is selected from group consisting of Bi, Ba, Sr, Ca, Y, La or a mixture of two or more thereof; D is selected consisting of V, Mo, Mn, Ti, Ta, Nb, W, Sb, Fe, Cr, Sn, Ce or mixture of two or more thereof; x is a number that is at least 1; y is the number of oxygens required to fulfill the valency of bismuth, A and D. The colorants are useful in providing colored compositions including: organic compositions, plastics, rubbers and inorganic compositions such as ceramics, enamels and coating compositions such as paints and printing inks. Prabhakar Rao and Reddy (Reference may be made to Dyes and Pigments, 2004, 63, 169-174) have reported yellow-orange colorants based on bismuth and rare earth oxides having the general formula (Bi2O3)1−x(RE2O3)x where RE is Y or Ce with a nominal composition; x=0.2 or 0.5 for Y; x=0.3 or 0.5 for Ce.
Yellow inorganic pigments consisting of double molybdates of cerium and of an alkali metal, of general formula CeM(MoO4)2 in which M denotes an alkali metal, preferably sodium has been described in an European patent (Reference may be made to EP0542343, May 19, 1993).
Japanese patent JP2003160742 discloses a process for a yellow cerium pigment having a general formula: ACxLn1−xMo2O8 wherein x ranges from 0 to 1; A in the composition is at least one element selected from the group consisting of Li, Na, K, Rb and Cs and Ln is at least one element selected from the group consisting of Y, La, Gd and Lu.
New yellow pigments without toxic metal to substitute the conventional CdS, PbCrO4 and Pb2Sb2O7, cerium thiosilicates have been reported (G. Gauthier, S. Jobic, M. Evain, H.-J. Koo, M.-H Whangbo, C. Fouassier, and R. Brec, Chem. Mater. 2003, 15, 828-837). However, there are still some problems in their synthesis processes. The heating conditions must be controlled exactly to synthesize the cerium thiosilicates and this method is too time consuming (minimum 13 days).
Although some alternative yellow pigments based on cerium oxide and other transition metal oxides have been reported, but their chromatic properties are not satisfactory for industrial use (further reference may be made to T. Masui, H. Tategaki, N. Imanaka, J. Mater. Sci. 2004, 39, 4909-4911; Imanaka et al. Chem. Lett. 2005, 34, 1322-1323; T. Masui, S. Furukawa, and N. Imanaka, Chem. Lett. 2006, 35, 1032-1033).